Schematic diagram of the sequence of events leading to the formation of a turbulent spot in terms of vortex filaments. (a) A jet-initiated pulse caused the vortex filament to develop an undulation. (b) The initially disturbed filament has been stretched and developed two more undulations laterally. The filament in front has started to induce disturbances in the filaments further downstream. (c) The initially disturbed filament has developed two more undulations laterally.
The dashed line in (c) (added by C. Delo) represents the heart-like outline of a fully-formed turbulent spot.
Reprinted from Figure 7 : "A visual study of turbulent spots", Journal of Flluid Mechanics, Vol. 104, 1981; pp. 387-405.© Cambridge University Press 1981. Reprinted with permission of Cambridge University Press.